This document provides details on E3SM development conventions and practices.

NOTE:

This document does not discuss the use of forks. If a developer understands and prefers to use a fork, it is recommended that they make use of forks.
 

Introduction

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important -- Items colored in red mean they are important, and should not be ignored.

one time -- Items colored in green are commands to be issued once per machine.

repo once -- Items colored in orange are commands to be issued once per local repository.

...

Within this diagram, blue dots have associated feature branches that are omitted for the sake of readability. However, each of them followed the same development cycle as the feature that is being focused on in the diagram. Also, red commits are made to next. Both next and master should only be modified by integrators or gatekeepers. 
 

Basic Development

Creating a new branch

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-new feature

New development should be carried out on a branch. New developments include the addition of a new feature (github-username/component/feature) or fixing a bug (github-username/component/bug-fix).

...

When creating a new feature branch, best practice suggests you should branch from a tested version of the code.   HOWEVER, while the model is in "version 0" mode, always start development from the HEAD of master.

Creating a new branch

...

-bug fix

Typically, a bug fix is applied to the commit that introduced the bug.  In that case, the branch creation has an additional argument:

...

• The bug existed in code imported from CESM such as in the initial version added to the repo.
• The bug was introduced prior to v0.3 (the version where the testing is working)

Creating a new branch

...

-on a maintenance branch

The E3SM repository will have one or more active maintenance branches usually to keep adding bug fixes and machine updates to old versions.   See Existing and Planned Tags and BranchesE3SM permanent branches for the list of maintenance branches and , their purpose and what kind of changes they will accept.

If one of the maintenance branches needs features added to it, make a branch from the head of the maintenance branch.  You do this by first checking out the branch by its name.the feature, machine update or bug fix you are working on needs to go on a maintenance branch, switch to that branch before making your feature branch.  For example:

git checkout maint-1.0

This will put your working directory at the head of the maintenance branch.  You can then start a new branch as you do for master.   When you make a pull request, specify the maintenance branch as the "base".

If you need to fix a bug on a maintenance branch and the bug was introduced by a commit on the branch, you can follow the same directions as for a bug-fix branch.

IMPORTANT:  We do not have "next" branches for maintenance branches to test multiple features against each other.  To compensate, all development on a maintenance branch should be done sequentially.  That is, one developer makes their branch, completes the feature, tests it and then has it integrated to the branch before the next development starts.   This should not be a problem because maintenance branches are not supposed to have heavy development.   You should always test feature branches but this is more important for maintenance since there is no "next" testing and you don't want to break the maintenance branch by integrating a broken feature.

Changing Branches

Creating a new branch will not If you are developing something for both a maintenance branch AND master, do the maintenance development first.  You can then ask the integrator to merge maint to master.  Or you can make 2 pull requests, one for maint and one for master.

If your development depends on something that was added to master AFTER the maintenance branch started, it can not be added to the maintenance branch.   Master "contains" the content on the maintenance branches (if they are not too old) and one can merge from maint to master.  We can not merge from master to maint.

IMPORTANT:  We do not have "next" branches for maintenance branches to test multiple features against each other.  To compensate, all development on a maintenance branch should be done sequentially.  That is, one developer makes their branch, completes the feature, tests it and then has it integrated to the branch before the next development starts.   This should not be a problem because maintenance branches are not supposed to have heavy development.   You should always test feature branches but this is more important for maintenance features since there is no "next" testing and you don't want to break the maintenance branch by integrating a broken feature.

Changing Branches

Creating a new branch will not change the current branch that is being worked on in the current working directory.

...

git checkout -b github-username/component/feature master

Utilizing the repository history

Seeing as all of the E3SM developers are stewards of the repository history, it would be useful for developers to understand how to make use of the history they are maintaining. 

The most useful command for making use of the history is the git log command. This command has an abundance of optional arguments and this second can be used to get an idea of what you can do with git log.

The basic usage of this command gives the equivalent of an svn log. Where you get a list of all commits and their commit messages. By default, git log only shows commits that are reachable in the history of the HEAD. Optional arguments can be used to look at different (or all) branches.

 One extremely useful version of git log gives a command line graph of the history.

git log --oneline --graph --decorate

git log can also only show local branches that match a certain naming convention. i.e.

 git log --branches=*pattern*

 The --branches option can be replaced with --remotes to only show remote tracking branches that match a pattern.

 A specific commit has three pieces of information. The first is the commit’s tree (i.e. the files and directories contained in the commit), the second is the commit’s message (the commit message issued when creating the commit), and the third is a list of the commit’s parents. There can be multiple parents for a single commit, which would imply the commit was a merge commit. Git stores the order of the parents, with the first parent always being the commit the merge was initiated from. This is useful to note, because within our workflow first parent commits on master should always be merge commits bringing in features or bug fixes. git log can be used to narrow your view to only see first parent commits as well, via:

git log --first-parent

 As you develop a new feature, you might migrate files from one place to another, or even rename the file. In order to have git show you all renames of a file, you can use the following command:

git log --name-only --follow -- path/to/file

 Any of these options can be combined to give more flexibility to exploring the history of a git repository and make the history more useful.

Committing new files and changes as you go

While working on a feature or a bug fix, you will be editing source code files. After editing a file, you might want to commit those changes to your local repository in order to checkpoint your work, or track the change you made. By default, git doesn’t track any files in the repository unless they are explicitly added to the repository. In order to add a file to the repository, you can use the following command:

git add path/to/file/in/repo

In addition to causing git to track the file, this will also stage all changes in the file. The next commit that is created will contain all changes in the stage by default. In order to commit only the changes in the staging area, the following command can be used:

git commit

NOTE:   The git add command will only stage the changes in the file at the time of the git add. If the file is subsequently modified after a git add, and then a git commit is issued, the commit will not contain changes between the git add and the git commit.   Use git status frequently to see the status of staged and un-staged files.

git commit will open up an editor where a commit message can be written.   Please refer to the Commit message template for more instructions on the E3SM commit message guidelines.

An alternative way of committing all changes to any files that are currently tracked by the repository is:

git commit -a

This will again open an editor for a commit message to be entered and the Commit message template should again be followed.
  

Advanced: Incorporating another branch on to your branch.

When developing a new feature, another developer might have created a feature you require for your work, or another developer might have made large changes to the interfaces you make use of. This section will describe how to incorporate those changes into your branch.

NOTE: 
This action can have negative consequences and cause issues in the future, so it is important to know what you’re doing prior to doing any of these.

Cherry-pick method

The first option for incorporating changes from another development line into your development history is via a cherry-pick. A cherry-pick will copy individual commits (or a range of commits) from one point in history onto the HEAD of your current development line. It should be used when the number of commits your future development efforts depend on are small (order 1-10). It can be used as follows:

git cherry-pick <commit-ish>

...

Committing new files and changes as you go

While working on a feature or a bug fix, you will be editing source code files. After editing a file, you might want to commit those changes to your local repository in order to checkpoint your work.  When you make a change that you want to commit you "add" it to the git stage with this command:

git add path/to/file/in/repo

The next commit that is created will contain all changes in the stage by default.  If you create a new file, also use git add to make the repo aware of it.

A commit can have changes from multiple files. In order to commit only the changes in the staging area, the following command can be used:

git commit

NOTE:   The git add command will only stage the changes in the file at the time you issue the command. If the file is subsequently modified you need to run git add again if you want those changes to be committed.   Use git status frequently to see the status of staged and un-staged files.

git commit will open up an editor where a commit message can be written.   Please refer to the Commit and PR message template for more instructions on the E3SM commit message guidelines.

It is tempting to do development with lots of little commits like this:

f8ef2b5f75 fix
d2e3616d55 progress 
e58c74745f more progress
0be335ead0 almost working

If you make "checkpoint" commits like this, be prepared to rewrite the commit history using "git rebase" (See https://git-scm.com/book/en/v2/Git-Tools-Rewriting-History) so that each commit has a substantive message as required in Commit and PR message template.  See 

Utilizing the repository history

Seeing as all of the E3SM developers are stewards of the repository history, it would be useful for developers to understand how to make use of the history they are maintaining. 

The most useful command for making use of the history is the git log command. This command has an abundance of optional arguments and this second can be used to get an idea of what you can do with git log.

The basic usage of this command gives the equivalent of an svn log. Where you get a list of all commits and their commit messages. By default, git log only shows commits that are reachable in the history of the HEAD. Optional arguments can be used to look at different (or all) branches.

 One extremely useful version of git log gives a command line graph of the history.

git log --oneline --graph --decorate

git log can also only show local branches that match a certain naming convention. i.e.

 git log --branches=*pattern*

 The --branches option can be replaced with --remotes to only show remote tracking branches that match a pattern.

 A specific commit has three pieces of information. The first is the commit’s tree (i.e. the files and directories contained in the commit), the second is the commit’s message (the commit message issued when creating the commit), and the third is a list of the commit’s parents. There can be multiple parents for a single commit, which would imply the commit was a merge commit. Git stores the order of the parents, with the first parent always being the commit the merge was initiated from. This is useful to note, because within our workflow first parent commits on master should always be merge commits bringing in features or bug fixes. git log can be used to narrow your view to only see first parent commits as well, via:

git log --first-parent

 As you develop a new feature, you might migrate files from one place to another, or even rename the file. In order to have git show you all renames of a file, you can use the following command:

git log --name-only --follow -- path/to/file

 Any of these options can be combined to give more flexibility to exploring the history of a git repository and make the history more useful.

Advanced: Incorporating another branch on to your branch.

When developing a new feature, another developer might have created a feature you require for your work, or another developer might have made large changes to the interfaces you make use of. This section will describe how to incorporate those changes into your branch.

NOTE: 
This action can have negative consequences and cause issues in the future, so it is important to know what you’re doing prior to doing any of these.

Cherry-pick method

The first option for incorporating changes from another development line into your development history is via a cherry-pick. A cherry-pick will copy individual commits (or a range of commits) from one point in history onto the HEAD of your current development line. It should be used when the number of commits your future development efforts depend on are small (order 1-10). It can be used as follows:

git cherry-pick <commit-ish>

This method is the easiest to use and one of the least likely to create issues in future development, and allows the most flexible review modifications. One downside to this method, however, is that the commits that are cherry-picked will now occur twice in the history.

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A pull request initiates a process to merge your branch into master. The process will be documented with the subsequent code review via discussion and comments on the PR.

1. Commit all your changes and push the branch to the shared repository, go to https://github.com/E3SM-Project/E3SM/branches and find your branch.  Click on locally.
2. Clean up any "checkpoint" commits.   After development is done, may have commits that look like:

   f8ef2b5f75 fix
   d2e3616d55 progress
   e58c74745f more progress
   0be335ead0 almost working

   We do NOT want these in our commit history.  Use git rebase (see https://git-scm.com/book/en/v2/Git-Tools-Rewriting-History) to rewrite the history of your branch and group these in to substantive commits that follow our Commit and PR message template
3. Push the cleaned-up branch to the github repo.
4. go to https://github.com/E3SM-Project/E3SM/branches and find your branch.  Click on  "New pull request".
5. You should verify base and compare of your pull request are correct.  The "base" is master and "compare" is your branch.
   Enter
   1. When updating submodules:
      1. submit a PR
   Title:  Make sure the title is 70 characters or less and explains the PR in a "verb noun"
   1. 1. to submodule's repo first: e.g. a PR to https://github.com/E3SM-Project/kokkos
         1. if you can't submit a PR, request "push" access from the developers of the submodule
      2. after the PR is merged, submit a PR to https://github.com/E3SM-Project/E3SM updating submodule hash
   1. Enter a PR Title:  Make sure the title is 70 characters or less and explains the PR in a "verb noun" format like "add cool new feature". Do not just use the branch name or what is filled in by default.  (If your branch has a single commit, the title of that commit will be the default, but if your branch has multiple commits, the branch name will be the default.)  You should edit the title to make sure it is descriptive enough. DO NOT include github issue #'s or JIRA tasks in the title. Hyperlinking does not work from the title.
   2. Write a PR Description.  
      1. In the "Write" field, provide a descriptive message of what all the commits in the PR will do together.  Do not start it with a phrase like "This PR".  Use the "verb noun" format again.   Repeat the title if necessary.    This description will be used as the commit message when the branch is merged so follow the Commit and PR message template guidelines.
      2. If a bug fix:  after the description, close any Github issue numbers for the bugs this commit fixes using keywords like "Fixes #123".  See https://help.github.com/articles/closing-issues-using-keywords/
      3. After the description and any issue numbers,  add a keyword indicating how the PR might change answers.  
         1. [BFB] or [non-BFB] or [CC]:  Add ONE of these keys to indicate if this commit will affect testing results to roundoff [non-BFB] or climate changing [CC]. Use [BFB] if-and-only-f commit is bit-for-bit and you know all the tests will pass without regenerating baselines.
         2. [FCC]:   Add [FCC] if the commit will change climate if a flag is activated
         3. [NML]:  Add [NML] if the commit introduces changes to the namelist.
      4. The reviewer will review the description as part of the pull request. The description will be used in the commit message used when the PR is merged to next and master.     The integrator may work with you to edit the PR description.  Do not include Confluence URL's in the PR description.
   3. Give PR label(s) ('Label' pull down menu).  Add a label for the component this PR involves.  Add the "bug fix" label if this is a bug fix.  Add the label(s) for BFB, non-BFB, CC, FCC, NML as appropriate.
   4. Assign a single Integrator to manage the PR ( use the 'Assignee' pull down menu). 
   5. If you want additional reviewers, add them using the Reviewer pull-down menu.  Anyone can be asked to review.
   6. When complete, click on "Create pull request".  This will start the code review and the process of moving this feature to master.  
   7. After pull request is created, add  the following in the Comments section:
      1. In the first comment, provide a link to the Design Document governing this PR.   See /wiki/spaces/CNCL/pages/25231511.
      2. In subsequent comments.  
         1. Provide information to aid the Integrator in running, testing, and validating the feature (but that is too specific to be included in the general PR description). 
         2. Say how the feature was tested.  Example:  "e3sm_developer on Titan passed".  If a test is expected to fail and required redoing baselines, state that and list the tests that fail.

   Your PR is not finished until it has been merged to master by the Integrator.

   This document can be used to help with pull request related issues.

   ...

   1. Check the github entry for the PR and make sure it has a good title and description, correct labels and a comment with a link to the Design Document.   A PR can not be merged to next or master unless it has a Design Document with Phase 1 and 2 completed. See /wiki/spaces/CNCL/pages/25231511.
   2. Look at the code changes either on github or using:  git log --reverse -p master.. on your checked out copy of the branch.
      1. Does new code hold up to visual inspection for code quality?    Look over code changes for glaring mistakes or code style issues (e.g. useful comments, reasonable subroutine lengths, new code in an existing file follows conventions of that file).
      2. Check to see if the description of the code changes in the PR match the actual changes.  Make sure nothing unrelated to the PR was committed accidentally.
      3. Although they can't be changed, see if commit messages on the branch follow the Commit and PR message template and let the developer know if they can not.  Consider asking the developer to squash commits to clean up the history.
         
      4. Have tests been added or suggested that exercise this feature?
      5. Does code run on all platforms after integration into next?

   ...

   Merges to master will be done locally by integrators.  See Integrator Guide for  for more info.

   Additional Topics

   
   This section will be used to house additional General information for people who are interested. It will attempt to clarify some of the comments made in the development reference by expanding a bit more on them. Developers can safely ignore this content, it's only provided for educational purposes.

   The list of "nevers":

   ...

   developers.

   The list of "nevers":

   • Never routinely merge from master to your feature branch.  See below for more info.
   • Never use "cherry-pick" on "next"
   • Never do production simulations using "next"
   • Never commit directly to "master" or "next".  Only use merge commits of branches.
   • Never rebase commits on your branch after it is merged to next.
   • Never start new development from "next".

   Changing the url for your remote

   After the rename of ACME-Climate/ACME to E3SM-Project/E3SM you should change url or "origin" in each clone.

   First verify that "origin" is still pointing to ACME-Climate/ACME.  It will unless you changed it yourself but you can check by running "git remote -v".

   You'll see output like:

   origin git@github.com:ACME-Climate/ACME.git (fetch)
   origin git@github.com:ACME-Climate/ACME.git (push)

   Change the URL for origin with this command:

   git remote set-url origin git@github.com:E3SM-Project/E3SM.git

   ...

   git remote set-url origin git@github.com:E3SM-Project/<repo name>.git

   Working with Remote repositories

    

   Git is a distributed content versioning system (DCVS). When you clone a repository, you make a local repository that is essentially a backup of whatever you cloned. When you commit, the commit only occurs in your local repository. In order to allow other people to make use of the commits you make, or to get your commits incorporated into master they need to be “pushed” to a remote. 

   A remote is a repository that you would like to communicate with, that is not in the local working directory. When you clone a repository, a default remote is created named “origin” that points to whatever you cloned. A remote is really just an alias to the location of another repository you either want to read or write with. 

   Before communicating with remotes, you might want to add or remove remotes. In order to add and remove remotes the git remote command can be used. The two uses are as follows:

   git remote add remote-name protocol:address/to/repo # Creates a remote

   git remote remove remote-name # Removes a remote

     

   In order to communicate with remotes, there are three actions. push, pull, and fetch.

   The push command can be used to write some commits from your local repository to a remote repository. For example: 

   git push <remote-name> <local-branch>:<remote-branch>

   can be used to write a local branch to a remote branch on the remote pointed to by the name provided. An explicit example would be: 

   git push E3SM-Project/E3SM feature:github-username/component/feature

    

   In this case, we’re writing the branch feature to the remote origin and changing its name to core/feature.

    git push is the command used when writing history to remotes. In order to read history from remotes, the git pull and git fetch commands can be used.

    The git fetch command will update the local repository with the most recent history from a specific remote, without modifying any local branches in the repository. It can be used as follows: 

   git fetch <remote-name>

     

   The git pull command is a combination of two operations. The first is a git fetch in order to update the history in the local repository of the branches that exist on the remote repository. The second is a git merge which merges the changes into the current working directory. It is only recommended to use this if you know what you’re doing. It can easily cause unexpected problems. It can be used as follows:

   ...

   Never Merge From Master

   You never need to "merge from master" to make the eventual merge of your feature branch "easier".   This is because, unlike svn, git is smart enough to ignore changes that have occurred in files you aren't working on when you finally merge your branch to master.

   This policy is to make it easier on developers - developers dont have to concern themselves with a hundred other things going on in E3SM and can focus solely on their feature.  The integrator will evaluate how this feature works with the latest version of E3SM when they merge the feature to 'next', before it is merged to 'master'.   We also don't want unnecessary merges from upstream since they just clutter and confuse the repository history and provide more opportunity to break things in the branch. However, there are cases where merging from master (or better, an upstream release) can make sense - such as a long lived development branch.  The rule of thumb for any merge is that if you can clearly describe what you are merging and why that merge is necessary for your branch to be completed (so that it can be merged upstream), then it's fine to merge.  But merges without such justification are generally undesirable.  (See discussion here )

   If you want to see how your feature works with the latest version of master, do a test merge.  Update your local version of master,  make a new branch called something like "testmaster" and check it out.   Merge your feature branch to this new branch (which is just a copy of master) and run your tests.   When you're done, you can delete the "testmaster" branch.

   ...

   • "next"
   • Never commit directly to "master" or "next".  Only commit to your feature branch.
   • Never rebase commits on your branch after it is merged to next (unless told to do so by integrator).
   • Never start new development from "next".

   Changing the url for your remote

   After the rename of ACME-Climate/ACME to E3SM-Project/E3SM you should change url or "origin" in each clone.

   First verify that "origin" is still pointing to ACME-Climate/ACME.  It will unless you changed it yourself but you can check by running "git remote -v".

   You'll see output like:

   origin git@github.com:ACME-Climate/ACME.git (fetch)
   origin git@github.com:ACME-Climate/ACME.git (push)

   Change the URL for origin with this command:

   git remote set-url origin git@github.com:E3SM-Project/E3SM.git

   verify with "git remote -v"
   
   origin git@github.com:E3SM-Project/E3SM.git (fetch)
   origin git@github.com:E3SM-Project/E3SM.git (push)
   
   If you have clones of other repositories that were on ACME-Climate, you'll need to update their URL's as well with similar command:

   git remote set-url origin git@github.com:E3SM-Project/<repo name>.git

   Working with Remote repositories

    

   Git is a distributed content versioning system (DCVS). When you clone a repository, you make a local repository that is essentially a backup of whatever you cloned. When you commit, the commit only occurs in your local repository. In order to allow other people to make use of the commits you make, or to get your commits incorporated into master they need to be “pushed” to a remote. 

   A remote is a repository that you would like to communicate with, that is not in the local working directory. When you clone a repository, a default remote is created named “origin” that points to whatever you cloned. A remote is really just an alias to the location of another repository you either want to read or write with. 

   Before communicating with remotes, you might want to add or remove remotes. In order to add and remove remotes the git remote command can be used. The two uses are as follows:

   git remote add remote-name protocol:address/to/repo # Creates a remote

   
   

   git remote remove remote-name # Removes a remote

     

   In order to communicate with remotes, there are three actions. push, pull, and fetch.

   The push command can be used to write some commits from your local repository to a remote repository. For example: 

   git push <remote-name> <local-branch>:<remote-branch>

   
   

   can be used to write a local branch to a remote branch on the remote pointed to by the name provided. An explicit example would be: 

   git push E3SM-Project/E3SM feature:github-username/component/feature

    

   In this case, we’re writing the branch feature to the remote origin and changing its name to core/feature.

    git push is the command used when writing history to remotes. In order to read history from remotes, the git pull and git fetch commands can be used.

    The git fetch command will update the local repository with the most recent history from a specific remote, without modifying any local branches in the repository. It can be used as follows: 

   git fetch <remote-name>

     

   The git pull command is a combination of two operations. The first is a git fetch in order to update the history in the local repository of the branches that exist on the remote repository. The second is a git merge which merges the changes into the current working directory. It is only recommended to use this if you know what you’re doing. It can easily cause unexpected problems. It can be used as follows:

   
   git pull <remote-name> <branch-name>

   
   

   Keeping your fork up-to-date.

   You will need to keep the master branch within your fork of E3SM up to date with main version.  Update as often as you want but always before you start new development or if you want to try a test PR on your fork.

   Use the "Sync" button on your fork.

   Avoid Routine Merges From Master

   You never need to "merge from master" to make the eventual merge of your feature branch "easier" or to "keep up".   Unlike svn, git is smart enough to ignore changes that have occurred in files you aren't working on when you finally merge your branch to master.  Also, this practice can cause more problems.

   This policy is to make it easier on developers - developers don't have to concern themselves with a hundred other things going on in E3SM and can focus solely on their feature.  The integrator will evaluate how this feature works with the latest version of E3SM when they merge the feature to 'next', before it is merged to 'master'.   We also don't want unnecessary merges from upstream since they just clutter and confuse the repository history and provide more opportunity to break things in the branch. 

   There was a discussion and several links on this topic here.   To summarize some of key points:

   • "frequent pulling of the \[master] into a development branch will add a certain amount of randomness to that branch; this randomness is not particularly helpful for somebody who is trying to get a feature working. It also increases the chances that another developer who ends up in the middle of the series while running a bisect operation will encounter unrelated bugs."
   • A branch has a specific purpose. A topic branch 'add-frotz' would be about adding a new 'frotz' feature and shouldn't do anything else.  When you merge from master, you declare that all the other unrelated changes done on 'master' in preparation for the next release somehow bring 'add-frotz' closer to the goal of the 'frotz' topic. That is usually not true
   • Unnecessary merges and similar repository clutter reduces the ability to summarize, audit, notice bugs in review, and find bugs after the fact.  Keeping clean history is not difficult.  It requires a little bit of discipline on the part of integrators and developers, but it's a small price to pay for the time saved and improved quality/reliability.

   There are some cases where merging from master (or better, a tagged version of master) can make sense.  The rule of thumb for any merge is that if you can clearly describe what you are merging and why that merge is necessary for your branch to be completed, then it's fine to merge.    For example, you might need a feature on master (some crucial functionality, not just a build system updates) to complete the feature on your branch.   Integrators may ask you to merge from master to help resolve conflicts during a PR integration.  But before merging from master, try one of the other ways to get features from other peoples development described in Incorporatinganotherbranchontoyourbranch.

   If you want to see how your feature works with the latest version of master, do a test merge with a throwaway branch.  Update your local version of master,  make a new branch called something like "testmaster" and check it out.   Merge your feature branch to this new branch (which is just a copy of master) and run your tests.   When you're done, you can delete the "testmaster" branch.  Repeat as necessary.

   If you want to know how an Integrator would merge a branch with conflicts they can't resolve, see Integrator Guide#Resolvingmergeconflicts

   On conflicts: While git is better at handing merges and branches and resulting conflicts, do not expect that you (or the Integrator) will never encounter a merge conflict. Merge conflicts still exist, and have to be dealt with. No version control tool will be able to automatically resolve a conflict when two people change the same section of code.   If two or more developers need to work on the same piece of code, or code that touches several routines, the way to avoid conflicts is to serialize the development or find the common part and do that first, then make new branches after those changes are committed. 

   Git branching model and avoiding conflicts:

   One of the main differences between git and svn is how history and branches are stored. In svn branches are virtual directories (i.e. svn copy ^/trunk branch_name) and the history is stored as a stack of revisions (providing the monotonically increasing revision number). The problem with this is that a branch isn't actually a branch. It doesn't contain the information about where it was created from or what has been "merged" into it. The same is true when a branch is merged back into trunk; there isn't a clear historical description of what has already been integrated into trunk. (NOTE: svn has put some effort into improving their branching and merging capabilities, but they are still not up-to-par with DVCS tools)

   However, in git the history is stored as a Directed Acyclic Graph (DAG), and branches are first-class citizens. The DAG structure allows any commit to determine which other commits it includes changes from (or which commits have been integrated into it). Using this history structure, git is able to ignore changes from commits that have already been integrated, reducing the number of merge conflicts that occur when bringing in a large set of changes.NOTE: While git is better at handing merges and branches, do not expect that you will never encounter a merge conflict. Merge conflicts still exists, and have to be dealt with. No version control tool will be able to automatically resolve a conflict when two people change the same line of code.

   Additional References on git merges

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